Positive electrode active material, and lithium secondary battery using same

ABSTRACT

A positive electrode active material for lithium secondary batteries disclosed herein comprises a lithium transition metal oxide of a layered structure, represented by formula Li 1+α Ni x Co y Mn z Ca β M γ O 2  (where −0.05≤α≤0.2, x+y+z+β+γ≅1, 0.3≤x≤≤0.7, 0.1≤y≤0.4, 0.1≤z≤0.4, 0.0002≤β≤0.0025, 0.0002≤β+γ≤0.02, and in a case where γ&gt;0, M is absent or represents one, two or more elements selected from the group consisting of Na, Mg, Al, Ti, V, Cr, Zr, Nb, Mo, Hf, Ta and W). The tap density of the positive electrode active material ranges from 1.8 to 2.5 g/cm 3 .

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a National Stage of International Application No.PCT/JP2014/051231, filed Jan. 22, 2014, claiming priority based onJapanese Patent Application No. 2013-011524, filed Jan. 24, 2013, thecontents of all of which are incorporated herein by reference in theirentirety.

TECHNICAL FIELD

The present invention relates to a positive electrode active materialfor lithium secondary batteries. The present invention relates furtherto a lithium secondary battery obtained by using the positive electrodeactive material.

BACKGROUND ART

Lithium secondary batteries boast smaller size, lighter weight andhigher energy density than existing batteries and exhibit improvedinput-output density. Accordingly, lithium secondary batteries arepreferably used as so-called portable power sources in personalcomputers or mobile terminals, and as high-output power sourcesinstalled in vehicles.

Such lithium secondary batteries are generally used in a state wherevoltage is controlled so as to lie within a predefined region (forinstance, 3.0 V to 4.2 V). However, the predefined voltage may in someinstances be exceeded and an overcharge state be thus brought about, ifmore current than usual is supplied to the battery, for instance due tosome malfunction. To cope with overcharge, therefore, batteries havebeen proposed that comprise a current interrupt device (hereafter,“CID”) that interrupts current when the pressure inside a battery casebecomes equal to or greater than a predetermined value. Generally, anonaqueous solvent or the like comprised in an electrolyte solutionundergoes electrolysis, and a gas is generated, when the battery entersan overcharge state. Upon detection of the gas, the CID cuts off thecharging path of the battery, and further overcharge is prevented as aresult. In order to activate the CID at a yet earlier stage ofovercharge it is therefore necessary to raise promptly the pressureinside the battery case, for instance through generation of a largeamount of gas.

As instances of prior art pertaining to this issue, for instance PatentLiterature 1 discloses the feature of adding a polymerizable compound(or polymer) to a nonaqueous electrolyte solution, and adding a carbondioxide generating agent to a positive electrode active material layer.By virtue of this feature, hydrogen ions are generated through reactionof the polymerizable compound in the electrolyte solution, duringovercharge; thereupon, the hydrogen ions react with the carbon dioxidegenerating agent, so that carbon dioxide can be generated as a result.In Patent Literature 1 a CID can thus be activated promptly as a result.

CITATION LIST Patent Literature

Patent Literature 1: Japanese Patent Application Publication No.2012-123955

Patent Literature 2: Japanese Patent Application Publication No.2003-267732

Patent Literature 3: Japanese Patent Application Publication No.2011-116580

SUMMARY OF INVENTION

Approaches to increasing the energy density of lithium secondarybatteries such as those used in, for instance, power sources for drivinga vehicle, are being studied as one way of improving the performance ofsuch batteries. Higher energy density can be realized, for instance, byadjusting the composition and properties (for instance, physicalproperties) of positive electrode active materials. Prior art citationsthat are relevant herein include, for instance, Patent Literature 2 and3.

However, studies by the inventors have revealed that a concern ofdelayed activation of the CID, during overcharge, arises depending on,for instance, the properties of the positive electrode active materialthat is used. More specifically, a concern arose in that the contactsurface area (specifically, reaction sites) between an electrode and anelectrolyte solution might be reduced, and, as a result, generation ofgas during overcharge be slowed down, due to, for instance, a reductionin the voids of the positive electrode active material layer in a casewhere the density of the positive electrode active material layer isincreased through adjustment of the particle size of the positiveelectrode active material. A further concern arose in that gas might notbe discharged smoothly from an electrode active material layer, due tonarrowing diffusion paths of the generated gas.

In the light of the above considerations, it is an object of the presentinvention to provide a positive electrode active material for producinga positive electrode that allows achieving both excellent batteryperformance (for instance, high energy density) and high reliabilityduring overcharge (overcharge resistance). A related object of thepresent invention is to provide a lithium secondary battery providedwith a (pressure-activated type) current interrupt device that isactivated as a result of a rise in battery internal pressure, such thatthe battery boasts both excellent battery performance and reliabilityduring overcharge.

The present invention provides a positive electrode active material forlithium secondary batteries. The positive electrode active material forlithium secondary batteries disclosed herein comprises a lithiumtransition metal oxide (hereafter referred to as “LNCMC oxide”) of alayered structure, represented by formulaLi_(1+α)Ni_(x)Co_(y)Mn_(z)Ca_(β)M_(γ)O₂. In the formula, α, x, y, z, βand γ are −0.05≤α≤0.2, x+y+z+β+γ≅1, 0.3≤x≤0.7, 0.1≤y≤0.4, 0.1≤z≤0.4,0.0002≤β≤0.0025 and 0.0002≤β+γ≤0.02. In a case where γ>0, M is absent orrepresents one, two or more elements selected from among sodium (Na),magnesium (Mg), aluminum (Al), titanium (Ti), vanadium (V), chromium(Cr), zirconium (Zr), niobium (Nb), molybdenum (Mo), hafnium (Hf),tantalum (Ta) and tungsten (W). The tap density of the positiveelectrode active material disclosed herein ranges from 1.8 g/cm³ to 2.5g/cm³.

A lithium transition metal oxide comprising Ni, Co and Mn as structuralelements is excellent in thermal stability, and exhibits a highertheoretical energy density than those of other oxides. A high batteryperformance (for instance, energy density and cycle characteristic) canbe realized as a result. Further incorporating Ca as a structuralelement allows generating quickly a large amount of gas duringovercharge. In addition, discharge paths of the generated gas can besecured, such that gas generated during overcharge can be dischargedquickly out of the electrode assembly, through adjustment of the tapdensity of the positive electrode active material so as to lie withinthe above range. As a result, the CID can be activated accurately at anearly stage of overcharge.

Therefore, the positive electrode active material disclosed hereinallows realizing both excellent battery performance (for instance, highenergy density and cycle characteristic) and reliability duringovercharge.

The tap density can be measured in accordance with the method specifiedin, for instance, JIS K1469 (2003), using an ordinary tapping-typedensity measuring device. In the present description, the term “lithiumsecondary battery” denotes generically a secondary battery in whichlithium ions are used as charge carriers (electrolyte ions), such thatcharge and discharge are accomplished through traffic of lithium ionsbetween a positive and a negative electrode.

In one preferred implementation disclosed herein, (a) an averageparticle size D₅₀ corresponding to a cumulative 50% from the fineparticle side, ranges from 5 μm to 9 μm; (b) and a particle size D₁₀corresponding to a cumulative 10% from the fine particle side, aparticle size D₉₀ corresponding to a cumulative 90% from the fineparticle side, and the average particle size D₅₀, satisfy the followingrelationship: (D₉₀−D₁₀)/D₅₀≤0.7; in a volume-basis particle sizedistribution measured on the basis of a laser diffraction/lightscattering method.

Suitable conductive paths can be formed between particles in a positiveelectrode active material that satisfies the above range of particlesize. As a result, it becomes possible to reduce the resistance (forinstance, charge transfer resistance) in the positive electrode activematerial layer, and to realize high battery performance. It becomes alsopossible to maintain proper voids within the positive electrode activematerial layer, and to elicit sufficient soaking by the nonaqueouselectrolyte solution. The above relational expression provides an indexthat denotes the spread of the particle size distribution. The spread ofthe particle size distribution is narrowed, i.e. the positive electrodeactive material particles are made homogeneous, so that the aboverelational expression is 0.7 or smaller. As a result, the voltage thatis applied to the particles is made homogeneous, and it becomes possibleto suppress local degradation of the positive electrode active materialaccompanying charge and discharge. Excellent battery performance (forinstance, energy density, input-output density, cycle characteristic)can be brought out during ordinary use, and a battery can be thereforesuitably realized in which a CID can be activated through promptgeneration of gas during overcharge.

In the present description, the term “average particle size” denotes aparticle size (also referred to as D₅₀, median size) corresponding to acumulative 50% from the fine particle side, in a volume-basis particlesize distribution measured on the basis of a particle size distributionmeasurement according to an ordinary laser diffraction/light scatteringmethod. Similarly to the above average particle size, the terms “D₁₀”and “D₉₀” denote respectively a cumulative 10% and a cumulative 90% fromthe fine particle side.

Preferably, the positive electrode active material is a hollow structurehaving a shell section made up of a lithium transition metal oxide of alayered structure, and a hollow section formed inside the shell section.

In the positive electrode active material of hollow structure, thediffusion distance of the lithium ions is short, and, accordingly,exchange of materials with the electrolyte solution (for instance,storage and release of lithium ions) can take place efficiently.Accordingly, a lithium secondary battery provided with such a positiveelectrode active material can exhibit a high input-output characteristic(in particular, high output density in a low SOC region, where iondiffusion into the active material is rate-limiting), and for instance adesired output can be achieved over a wide SOC range.

Studies by the inventors have revealed that gas generated duringovercharge may in some instances fail to be discharged smoothly, out ofthe positive electrode active material layer, in cases where an ordinarypositive electrode active material of hollow structure is used. Thereactivity, during overcharge, of the positive electrode active materialdisclosed herein is however enhanced by comprising Ca as a structuralelement. Accordingly, the CID can be activated at an early stage throughprompt generation of a large amount of gas, even when the positiveelectrode active material is set to have a structure (for instance,hollow structure) that is close to hollowness. That is, excellentbattery performance (for instance, input-output density) and reliabilityduring overcharge can both be achieved at a yet higher level.

In the positive electrode active material of hollow structure,preferably, the thickness of the shell section based on an electronmicroscope observation is 2 μm or smaller. A yet higher input-outputcharacteristic can be realized by keeping small the thickness of theshell section and/or the primary particle size. Preferably, thethickness of the shell section based on an electron microscopeobservation is 0.1 μm or greater. Prescribing such a thickness allowssecuring higher durability against stress that is incurred duringproduction or use of the battery, and against expansion and contractionof the positive electrode active material accompanying charge anddischarge. Therefore, excellent performance can be realized stably, overlong periods of time, in a battery that utilizes a positive electrodeactive material that satisfies the above thickness of shell section.

In the present description, the term “positive electrode active materialof hollow structure” denotes a positive electrode active materialwherein a proportion (particle porosity described below) taken up by ahollow section in an apparent cross-sectional area of the activematerial in a cross-section resulting from cutting the positiveelectrode active material at a random position, is 5% or higher. In thepresent description, the term “SOC” denotes the state of charge of thebattery, taking as a reference the voltage range over which the batteryis normally used. For instance, “SOC” refers to the state of chargetaking as a reference a rated capacity measured under conditions ofvoltage across terminals (open circuit voltage (OCV)) of 4.1 V (upperlimit voltage) to 3.0 V (lower limit voltage).

Preferably, such positive electrode active material particles have athrough-hole that runs through the shell section (the hollow structurehaving a through-hole in the shell section is also referred to as“pierced hollow structure” thereafter). In a pierced positive electrodeactive material of hollow structure, the electrolyte solution seepsreadily into the hollow section, and materials can be exchangedefficiently with the electrolyte solution in the hollow structure.Therefore, the output characteristic can be improved (in particular,output characteristic in a low SOC region), and desired output can beachieved over a wider SOC range. Accordingly, a battery provided withsuch a positive electrode active material allows achieving, at a higherlevel, both battery performance (for instance, input-outputcharacteristic) and reliability during overcharge.

Preferably, a crystallite size r of the positive electrode activematerial, based on X-ray diffraction, ranges from 0.05 μm to 0.2 μm. Byvirtue of this feature, increases in resistance can be kept small, andit becomes possible to achieve, at a yet higher level, both batteryperformance (in particular, output characteristic) and reliabilityduring overcharge.

The present invention provides a lithium secondary battery having aconfiguration wherein an electrode assembly comprising a positiveelectrode and a negative electrode, and a nonaqueous electrolytesolution, are accommodated within a battery case. The battery casecomprises a current interrupt device that is activated when the internalpressure of the battery case rises. The positive electrode comprises anyone of the positive electrode active materials disclosed herein. Thelithium secondary battery disclosed herein affords high reliabilityduring overcharge, while preserving good battery performance. Forinstance, the energy density and input-output density becomes higher, adesired output can be achieved over a wide SOC range, and the CID can beproperly activated. By exploiting such features, the present inventioncan therefore be suitably used, for instance, as a power source (drivingpower source) of a vehicle.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a perspective-view diagram illustrating schematically theoutline of a lithium secondary battery according to one embodiment;

FIG. 2 is a diagram illustrating schematically a cross-sectionalstructure of FIG. 1 along line II-II;

FIG. 3 is a schematic diagram illustrating the configuration of a woundelectrode assembly of FIG. 2;

FIG. 4 is a diagram illustrating schematically a cross-sectionalstructure of a positive electrode active material according to oneembodiment;

FIG. 5 is a SEM observation image of a positive electrode activematerial according to one embodiment;

FIG. 6 is a cross-sectional SEM observation image of a positiveelectrode active material according to one embodiment;

FIG. 7 is a graph illustrating a correlation of battery characteristicsand Ca addition ratio in a lithium transition metal oxide; and

FIG. 8 is a graph illustrating a correlation between tap density andcharacteristics of a positive electrode active material.

DESCRIPTION OF EMBODIMENTS

Preferred embodiments of the present invention will be explained nextwith reference to accompanying drawings. Any features other than thefeatures specifically set forth in the present description (forinstance, the composition or properties of the positive electrode activematerial) and which may be necessary for carrying out the presentinvention (for instance, a construction method of an ordinary battery)can be regarded as instances of design matter for a person skilled inthe art on the basis of known techniques in the technical field inquestion. The present invention can be realized on the basis of thedisclosure in the present description and on the basis of commontechnical knowledge in the technical field in question. In the drawingsbelow, members and sites that elicit identical effects are denoted withidentical reference numerals, and a recurrent explanation thereof willbe omitted or simplified. Further, the dimensional relationships(length, width, thickness and so forth) do not necessarily reflectactual dimensional relationships.

<<Positive Electrode Active Material>>

Herein, the positive electrode active material (shell section ofpositive electrode active material particles in the positive electrodeactive material of hollow structure described below) comprises a lithiumtransition metal oxide having a layered crystal structure (typically alayered rock salt-type structure belonging to a hexagonal system)represented by formula Li_(1+α)Ni_(x)Co_(y)Mn_(z)Ca_(β)M_(γ)O₂.Including Ca among the structural elements allows forming a compound ofLi and Ca. According to studies by the inventors, forming such acompound allows suppressing polymerization of a nonaqueous electrolytesolution, and reducing the amount of alkalis (for instance, lithiumhydroxide (LiOH)) at the positive electrode active material surface. Asa result, it becomes possible to enhance reactivity of a gas generatingagent during overcharge, and to generate quickly a greater amount ofgas, as compared with an instance where, for example, the above compoundis incorporated as an additive (gas generating agent) in the nonaqueouselectrolyte solution.

The feature “comprising a lithium transition metal oxide” indicates thatthe positive electrode active material is substantially made up of theabove oxide, while allowing for the presence of incidental impurities.

From the perspective of suppressing an increase in resistance, the abovecc is a real number that satisfies −0.05≤α≤0.2. Further, x, y, z, β andγ are real numbers that satisfy x+y+z+β+γ≅1 (typically, 0.95 to 1.02,for instance 1 to 1.02, and preferably 1). Herein, x, y and z are realnumbers such that, typically, 0.98≤x+y+z≤0.9998, where x is a realnumber that satisfies 0.3≤x≤0.7, y is a real number that satisfies0.1≤y≤0.4, and z is a real number that satisfies 0.1≤z≤0.4. In apreferred implementation, x and z are roughly similar (for instance, thedifference between x and z is 0.1 or less), i.e. the amount of Ni andthe amount of Mn are substantially similar (for instance, the differencebetween the amount of Ni and the amount of Mn is 10% or less). Inanother preferred implementation, x, y and z are roughly similar (forinstance, differences among x, y and z are 0.1 or less), i.e. the amountof Ni, the amount of Co and the amount of Mn are roughly similar (forinstance, differences between the Ni amount, Co amount and Mn amount are10% or less). An LNCMC oxide of such composition exhibits excellentthermal stability and battery characteristics, and is accordinglypreferable.

The above β and γ are the proportions of substitutional elements in theLNCMC oxide, and are real numbers that satisfy 0.0002≤β+γ≤0.02, from theviewpoint of maintaining high energy density. Specifically, β is a realnumber that satisfies 0.0002≤β≤0.0025 (typically, 0.0005≤β≤0.002, forinstance 0.001≤β≤0.002), and γ is a real number that satisfies0≤γ≤0.0198. Preferably, the LNCMC oxide having such a compositionexhibits excellent thermal stability and battery characteristics. In acase where γ>0, M is one, two or more elements selected from among Na,Mg, Al, Ti, V, Cr, Zr, Nb, Mo, Hf, Ta and W.

In the present description, for convenience, the composition ratio of O(oxygen) is represented as 2 in the chemical formula that denotes alithium transition metal oxide, but this numerical value is not to beinterpreted in a strict manner, and a certain degree of compositionfluctuation is allowable (typically, in the range from 1.95 to 2.05).

The tap density of the positive electrode active material disclosedherein is preferably 1.8 g/cm³ or higher (preferably, 1.85 g/cm³ orhigher, and more preferably 1.88 g/cm³ or higher). When the above rangesare satisfied, it becomes possible to increase the proportion of thepositive electrode active material in the positive electrode activematerial layer, i.e. to increase the battery capacity (energy density)per unit volume. The tap density of the positive electrode activematerial is preferably 2.5 g/cm³ or lower (preferably, 2.45 g/cm³ orlower, more preferably 2.41 g/cm³ or lower). When the above ranges aresatisfied, it becomes possible to retain proper voids within thepositive electrode active material layer; as a result, the activematerial layer is readily impregnated with an electrolyte solution, andthe diffusion resistance of lithium ions within the positive electrodeactive material layer can be kept low. Storage and release of lithiumions can take place as a result more efficiently, and, in particular,the output characteristic can be enhanced (particularly the outputcharacteristic in a low SOC region). In addition, gas generated duringovercharge can be discharged quickly out of the electrode assembly, andthe CID can be activated promptly.

The average particle size (secondary particle size) of the positiveelectrode active material may be, for instance 3 μm or greater, from theviewpoint of securing suitable voids within the positive electrodeactive material layer. In the positive electrode active material havingthe hollow structure described below, preferably, the average particlesize is in particular 5 μm or greater (typically, 5.5 μm or greater).Studies by the inventors have revealed that when the average particlesize is excessively small, the volume of the hollow section is likewisesmall, and, as a result, the effect of enhancing battery performance mayin some instances be poor. In terms for instance of productivity, theaverage particle size is preferably about 10 μm or smaller, and morepreferably, for instance, about 9 μm or smaller (typically, 8.5 μm orsmaller). Good battery performance can be realized, yet more stably,when the above ranges are satisfied. The average particle size and theabove-described tap density exhibit substantially a correlationrelationship within the range of suitable average particle sizedisclosed herein. If there is no difference in starting materials orproduction methods, then a trend is ordinarily observed whereby thelarger the average particle size, the higher the tap density is.Specifically, by prescribing the average particle size to lie thus inthe above ranges it becomes possible to suitably realize a battery thatallows combining, at a higher level, both battery performance (forinstance, energy density and input-output density) and reliabilityduring overcharge.

An index (D₉₀−D₁₀)/D₅₀ that denotes the spread of the particle sizedistribution, and that is expressed using the average particle size D₅₀,a particle size D₁₀ corresponding to a cumulative 10% from the fineparticle side, and a particle size D₉₀ corresponding to a cumulative 90%from the fine particle side, is preferably 0.7 or smaller (typically,0.6 or smaller, for instance 0.55 or smaller, or preferably in the range0.4 to 0.55). By setting a small spread of particle size distribution,namely 0.7 or smaller, i.e. by prescribing a uniform granularity of thepositive electrode active material, it becomes possible to render morehomogeneous the voltage that is applied to the positive electrode activematerial particles and to suppress local degradation of the positiveelectrode active material accompanying charge and discharge. Therefore,a high-durability battery can be realized that affords high batteryperformance (for instance, energy density) stably over long periods oftime.

The lithium transition metal oxide disclosed herein has a layeredcrystal structure (typically, a layered rock salt-type structurebelonging to a hexagonal system). The layers are laid along the (003)plane direction, and lithium ions are deemed to move, along spacesbetween the planes, through the interior of the positive electrodeactive material particles. Accordingly, a crystallite size r along the(003) plane direction of the positive electrode active material ispreferably 0.05 μm or greater (typically, 0.06 μm or greater, forinstance 0.08 μm or greater), and 0.2 μm or smaller (typically, 0.15 μmor smaller, for instance 0.11 μm or smaller). By virtue of that featureit becomes possible to keep resistance low in charge and dischargecycles, in particular at a high rate, and to maintain a high capacity.Therefore, both battery performance (in particular, outputcharacteristic and energy density) and reliability during overcharge canbe achieved at a yet higher level.

The crystallite size r can be calculated on the basis of a value ofdiffraction peaks (half width) obtained by X-ray diffractionmeasurements using CuKα rays, on the basis of Expression (1) below.r=(0.9×λ)/(β×COS θ)  Expression (1)The meanings of r, X, β and θ are as follows. The Bragg angle θ of thediffraction lines is set to lie in the range 17.9° to 19.9°, and thevalue of the half-width β at that θ is substituted in Expression (1)

r: crystallite size

λ: X-ray wavelength (CuKα)

β: spread (rad) of diffraction peak derived from the crystallite

θ: Bragg angle of the diffraction line.

In a preferred implementation, the positive electrode active materialadopts the form of particles of hollow structure, having a shell sectionmade up of a lithium transition metal oxide of a layered structure, anda hollow section (cavity) formed inside the shell. Typically, suchparticle shape is, for instance, a substantially spherical or somewhatdistorted spherical shape. Examples of particles that are comparable tosuch particles of hollow structure include, for instance, ordinaryparticles of porous structure (solid structure). The term “porousstructure” denotes herein a structure (sponge-like structure) in which asolid portion and void portions are mixed throughout the particle. Inthe positive electrode active material particles of hollow structuredisclosed herein, the solid portion is localized towards the shellsection, with a space being clearly formed in the hollow section.Further, the space taken up by the hollow section is larger than thegaps that yield secondary particles, and, accordingly, the positiveelectrode active material particles of hollow structure are clearlydifferent, in structural terms, from positive electrode active materialparticles having a porous structure.

Such particles of hollow structure tend to collapse more readily, duefor instance to stress load, than particles of solid structure. When forinstance the hollow structure collapses due to stress load or the like,the voids in the positive electrode active material layer become fewer,and a concern arises in that gas generated during overcharge may fail tobe discharged smoothly through the active material layer. The positiveelectrode active material disclosed herein, however, comprises Ca as astructural element, so that a large amount of gas can be promptlygenerated thereby during overcharge. Generation of a large amount of gasat an early stage of overcharge allows thus implementing a CID stably.

FIG. 4 illustrates schematically a representative structure of suchpositive electrode active material particles. Positive electrode activematerial particles 110 are particles of hollow structure, having a shellsection 115 and a hollow section 116. The shell section 115 has aconfiguration resulting from aggregation of primary particles 112 into aspherical shell-like shape. In one preferred implementation, thecross-section of the shell section 115 exhibits a form wherein primaryparticles 112 are contiguous to each other (as multiple spheres), in anobservation image obtained using an electron microscope (for instance, ascanning electron microscope (SEM). That ring-like section may adopt aform in which the primary particles 112 are contiguous to each other, ina single layer, over the entirety of the shell section 115, or a formhaving a portion in which the primary particles 112 are contiguouslystacked on each other in two or more layers (multilayer). Preferably,the number of layers of the primary particles 112 at portions where thelatter are contiguous to each other is about 5 or fewer (for instance, 2to 5), and more preferably about 3 or fewer (for instance, 2 to 3). Thepositive electrode active material particles 110 according to onepreferred implementation are configured to adopt a form wherein theprimary particles 112 are contiguous to each other substantially in asingle layer, in the entirety of the shell section 115.

The positive electrode active material particles (secondary particles)110 having such a configuration exhibit less aggregation of the primaryparticles 112 than in the case of positive electrode active materialparticles that have a dense structure, with no cavities in the interior.As a result, the grain boundaries inside the particles are fewer innumber (and accordingly the diffusion distance of lithium ions shorter),which makes for a higher diffusion rate of lithium ions into theparticles. The output characteristic can therefore be effectivelyenhanced in a lithium secondary battery having such positive electrodeactive material particles 110 with few grain boundaries. For instance, alithium secondary battery can be constructed that exhibits good outputalso in a low SOC region (for instance, SOC of 30% or less) for whichion diffusion into the active material is rate-limiting.

As used herein, the term “primary particles” denotes particles thegeometry whereof can be considered to be outwardly that of unitparticles (ultimate particles). In the positive electrode activematerial disclosed herein, the primary particles are typicallyaggregates of crystallites of a lithium transition metal oxide. Theshape of the positive electrode active material can be observed, forinstance, using a FE-SEM “Hitachi ultra-high resolution field-emissionscanning microscope S5500” by Hitachi High-Technologies Corporation.

A major axis L1 of the primary particles 112 that make up the positiveelectrode active material particles 110 is 1 μm or smaller, and mayrange for instance from about 0.1 μm to 1 μm. Findings by the inventorshave revealed that the cycle characteristic of the battery may tend toworsen when the major axis L1 of the primary particles 112 isexcessively small. Such being the case, a positive electrode activematerial having a L1 of 0.2 μm or greater is preferable; morepreferably, L1 is 0.3 μm or greater, and yet more preferably, 0.4 μm orgreater. When L1 is excessively large, on the other hand, thereincreases the distance from the surface of the crystals up to theinterior (central section of L1) (i.e., the diffusion distance of thelithium ions), and, accordingly, ion diffusion into the crystal slowsdown, and the output characteristic tends to drop (in particular, theoutput characteristic in a low SOC region). Given the aboveconsiderations, L1 is 1 μm or smaller, typically 0.8 μm or smaller, andpreferably, for instance, 0.75 μm or smaller. In one preferredimplementation, the major axis L1 of the primary particles ranges from0.2 μm to 1 μm (for instance, from 0.3 μm to 0.8 μm). The major axis L1of the primary particles 112 and the value of the crystallite size rdescribed above exhibit roughly a correlation relationship. A trend isgenerally observed whereby the larger the major axis L1, the larger thecrystallite size r is as well.

The major axis L1 of the primary particles 112 can be measured on thebasis of observation images, by electron microscopy (for instance, SEM),of the particle surface of the positive electrode active materialparticles (secondary particles) 110. To measure the primary particlesize of the positive electrode active material particles comprised inthe positive electrode active material layer, it suffices to observe,under an electron microscope, the surface of the positive electrodeactive material particles that appear on a cross-section of the slicedactive material layer. For instance, suitable primary particles 112 areidentified, in such electron micrographs, in order to define the majoraxis L1. Specifically, a plurality of primary particles 112 is capturedon an electron micrograph of the particle surface of the positiveelectrode active material particles (secondary particles) 110, and hencea plurality of the primary particles 112 is extracted in descendingorder of display surface area on the electron micrograph. As a result,it becomes possible to extract primary particles 112 the capturedoutline of which runs substantially along the longest major axis L1, onthe electron micrograph of the particle surface. The longestlongitudinal axis length of the extracted primary particles 112 may beset herein as the major axis L1.

The thickness of the shell section 115 (portion resulting fromaggregation of primary particles into a spherical shell) in the positiveelectrode active material particles 110 is 2 μm or smaller, preferably1.8 μm or smaller, and yet more preferably 1.5 μm or smaller. Thesmaller the thickness of the shell section 115, the more readily thelithium ions are released from the interior of the shell section 115(central section in the thickness) during charging, and the more readilythe lithium ions are absorbed into the shell section 115 duringdischarge. It becomes possible as a result to increase, underpredetermined conditions, the amount of lithium ions per unit mass thatcan be stored in and released from the positive electrode activematerial particles, and to reduce resistance at those times where thepositive electrode active material particles store or release lithiumions. A lithium secondary battery that utilizes such positive electrodeactive material particles 110 can exhibit therefore an excellent outputcharacteristic.

The lower limit of the thickness of the shell section 115 is notparticularly restricted, but, ordinarily, is preferably about 0.1 μm orgreater. Prescribing the thickness of the shell section 115 to be 0.1 μmor greater allows securing higher durability against for instance stressthat is incurred during production or use of the battery, and againstexpansion and contraction of the positive electrode active materialaccompanying charge and discharge. The performance of the lithiumsecondary battery can be stabilized thereby, while suitably securingmoreover diffusion paths for the electrolyte solution and gas.Therefore, the thickness of the shell section 115 ranges preferably fromabout 0.1 μm to 2 μm, more preferably from 0.2 μm to 1.8 μm, andparticularly preferably from 0.5 μm to 1.5 μm, in terms of combining, ata high level, an internal resistance lowering effect, durability, andreliability during overcharge.

The thickness of the shell section 115 denotes herein the average valueof a shortest distance T(k) from any position k of an inner surface 115a (a portion corresponding to a through-hole 118 is not included in theinner surface 115 a) of the shell section 115 up to an outer surface 115b of the shell section 115, in a cross-sectional electron micrograph ofthe positive electrode active material or a material that comprises thepositive electrode active material particles 110. More specifically, theshortest distance T(k) is the arithmetic average value of values ofshortest distance T(k) worked out for a plurality of positions at theinner surface 115 a of the shell section 115. In this case, thethickness T of the shell section 115 converges to a average value, sothat the thickness of the shell section 115 can be evaluated properly asa result, as there increases the number of points for which the shortestdistance T(k) is worked out. Preferably, the thickness of the shellsection 115 is worked out on the basis of, ordinarily, at least 10 (forinstance, 20 or more) positive electrode active material particles 110.Preferably, the thickness of the shell section 115 is worked out on thebasis of an electron micrograph of cross-sections for at least any 3sites (for instance, 5 sites or more) of the positive electrode activematerial particles.

Preferably, the positive electrode active material particles 110 havethe through-hole 118 that runs through the shell section 115 and thatconnects spatially thereby the hollow section 116 and the exterior(exterior of the particles 110). Thanks to the presence of thethrough-hole 118, the electrolyte solution can move readily into and outof the hollow section 116. The electrolyte solution inside the hollowsection 116 can thus be appropriately replaced. As a result, dry-out dueto shortage of electrolyte solution inside hollow section 116 becomesunlikelier, and the primary particles 112 that face the hollow section116 can be utilized more actively in charge and discharge. In such aconfiguration, the thickness of the shell section 115 described above is2 μm or less; as a result, lithium ions diffuse quickly into thecrystals, while the electrolyte solution can be brought efficiently intocontact with the primary particles 112. The output characteristic of thelithium secondary battery (in particular, output characteristic in a lowSOC region) can be further enhanced thereby. Studies by the inventorshave revealed that positive electrode active material particles havingthrough-holes exhibit generally a tendency whereby gas generated duringovercharge is not readily discharged smoothly out of the positiveelectrode active material layer. Thanks to the positive electrode activematerial disclosed herein, however, the CID can be activated at an earlystage, also in such a structure, so that high reliability duringovercharge can be thus realized.

Preferably, the number of through-holes 118 in the positive electrodeactive material particles 110 ranges from about 1 to about 10 (forinstance, 1 to 5), as an average per particle of the positive electrodeactive material particle 110. The hollow structure may be difficult topreserve if the average number of through-holes is excessively large.Thanks to the positive electrode active material particles 110 havingthe preferred average number of through-holes disclosed herein, thebattery performance-enhancing effect (for instance, output-enhancingeffect) derived from having the pierced hollow structure can be elicitedappropriately and stably, while securing the strength of the positiveelectrode active material particles 110.

An opening width h of the through-hole 118 may be of about 0.01 μm orgreater, as an average value of a plurality of positive electrode activematerial particles. The opening width h of the through-hole 118 denotesherein the span length of the portion of through-hole 118 at which thepath from the exterior of the positive electrode active materialparticle 110 to up to the hollow section 116 is narrowest. When theopening width of the through-hole 118 is equal to or greater than 0.01μm in average, the through-hole 118 can be made to function moreeffectively as a flow passage of electrolyte solution. As a result, itbecomes possible to bring out the effect of enhancing the batteryperformance of the lithium secondary battery more properly.

In a case where one positive electrode active material particle 110 hasa plurality of through-holes 118, the opening width of the through-holehaving the largest opening width, from among the plurality ofthrough-holes 118, may be used as the opening width of the activematerial particles 110. The opening width h of the through-hole 118 is 2μm or smaller in average, more preferably 1 μm or smaller in average,and yet more preferably 0.5 μm or smaller in average.

Characteristic values such as the average number of through-holes,average opening size and the like can be grasped for instance throughobservation of the cross-section of the positive electrode activematerial particles by electron microscopy. For instance, the positiveelectrode active material particles or a material comprising the activematerial particles may be embedded in an appropriate resin (preferably,a thermosetting resin), after which the sample is cut to an appropriatecross-section, and the resulting cut cross-section is observed byelectron microscopy while being polished little by little.Alternatively, the above characteristic value can be calculated throughstatistical processing of the results of electron microscope observationof a single cross-section or of a comparatively small number ofcross-sections, for instance about 2 to 10 cross-sections, since theorientation of the positive electrode active material particles in thesample can ordinarily be assumed to be substantially random.

In one preferred implementation, the shell section 115 is sintereddensely at portions other than the through-hole 118 (typically, denselyenough so as not to allow an ordinary electrolyte solutions to passtherethrough). By virtue of the positive electrode active materialparticles 110 having such a structure, the sites at which theelectrolyte solution can flow between the exterior of the particles 110and the hollow section 116 can be limited to a given number ofthrough-holes 118. A particularly advantageous effect can be elicited asa result, for instance, in positive electrode active material particlesthat are used in batteries provided with a wound electrode assembly.Upon repeated charge and discharge in a battery provided with a woundelectrode assembly, the electrolyte solution is squeezed out from theelectrode assembly (in particular, positive electrode material layer) asa result of the expansion and contraction of the positive electrodeactive material particles accompanying charge and discharge; inconsequence, the electrolyte solution becomes insufficient in part ofthe electrode assembly, and battery performance (for instance, theinput-output characteristic) may drop. By virtue of the positiveelectrode active material particles 110 having the above configuration,the outflow of electrolyte solution from inside the hollow section 116is blocked at portions other than the through-hole 118, and hence itbecomes possible to effectively prevent, or reduce, shortage (dry-out)of electrolyte solution in the positive electrode active material layer.Such positive electrode active material particles have high shaperetention (i.e. are not prone to collapsing, which can be reflected inthat, for instance, the particles have high average hardness and highcompressive strength). Therefore, good battery performance can berealized, yet more stably.

The positive electrode active material particles 110 have a hollowstructure such that particle porosity is 5% or higher, and havepreferably a hollow structure such that particle porosity is 10% orhigher (for instance, 15% or higher). The advantages of the hollowstructure may in some instances fail to be readily brought out, to asufficient degree, when the particle porosity is excessively small.Particle porosity may be 20% or higher (typically 23% or higher,preferably 30% or higher). The upper limit of particle porosity is notparticularly restricted, but, ordinarily, is suitably set to 95% orlower (typically 90% or lower, for instance 80% or lower), from theviewpoint of durability of the positive electrode active materialparticles (for instance, performance in terms of preserving a hollowstructure against compressive stress or the like that may act on theparticles during production or use of the battery). The hollow structurecan be suitably maintained, and high input-output characteristics can bebrought out in a sustained manner, by prescribing the above ranges.

Herein, the term “particle porosity” denotes the proportion of a hollowsection within an apparent cross-sectional area of the positiveelectrode active material, in an average of cross-sections that are cutat random positions of the active material. This proportion can begrasped from observed images, by electron microscopy, of appropriatecross-sections of the positive electrode active material particles orthe material comprising the positive electrode active materialparticles. Particle porosity can be grasped from electron micrographs ofsuch cross-sections, similarly to the way in which the above averagenumber of through-holes, average opening size and so forth are grasped.In the electron micrographs of the cross-sections, the shell section115, the hollow section 116 and the through-hole 118 of the positiveelectrode active material particles can be distinguished from each otheron the basis of differences in color tone or shading. Therefore, a ratio(C_(V)/C_(T)) is obtained between a surface area C_(V) taken up by thehollow section 116 of the positive electrode active material particles110 and the cross-sectional area C_(T) apparently taken up by thepositive electrode active material particles 110, for a plurality ofpositive electrode active material particles 110 depicted in anarbitrary cross-sectional observation image of the above sample. Thecross-sectional area C_(T) apparently taken up by the positive electrodeactive material particles denotes herein the cross-sectional areaoccupied by the shell section 115, the hollow section 116 and thethrough-hole 118 of the positive electrode active material particles.The proportion (i.e. particle porosity) taken up by the hollow section116 within the apparent volume of the positive electrode active materialparticles can be worked out approximately on the basis of such ratio(C_(V)/C_(T)).

Preferably, there is calculated the arithmetic mean of values of theabove ratio (C_(V)/C_(T)) for electron micrographs of a plurality ofarbitrary cross-sections of the above sample. The arithmetic averagevalue of the ratio (C_(V)/C_(T)) converges as there increases the numberof cross-sectional observation images on the basis of which such ratio(C_(V)/C_(T)) is worked out, and thus the number of positive electrodeactive material particles that serve as a basis for calculating theratio (C_(V)/C_(T)). Preferably, particle porosity is ordinarily workedout on the basis of at least 10 (for instance, 20 or more) positiveelectrode active material particles. Preferably, particle porosity isworked out on the basis of an observation image of at least 3 (forinstance, 5 or more) arbitrary cross-sections of the sample.

The average hardness of the positive electrode active material particles110 ranges preferably from about 0.5 MPa to 100 MPa. By comprising Ca asa structural element, the pierced positive electrode active materialparticles of hollow structure disclosed herein can be harder (havehigher average hardness) and exhibit better shape stability thanpositive electrode active material particles of ordinary porousstructure (solid structure). Positive electrode active materialparticles of hollow structure and high average hardness (in other words,high shape retention) allow thus realizing a battery that can deliverhigh performance more stably.

As used herein, the term “average hardness” denotes a value obtained asa result of a dynamic micro-hardness measurement performed underconditions of load rate 0.5 mN/sec to 3 mN/sec, using a flat diamondindenter having a diameter of 50 μm. For instance, a micro-hardnesstester, model “MCT-W500” by Shimadzu Corporation, can be used for suchdynamic micro-hardness measurement. The arithmetic average value of thehardness of the active material converges as the above hardnessmeasurement is performed for a greater number of positive electrodeactive material particles. Preferably, an arithmetic average value basedon at least 3 (preferably, 5 or more) positive electrode active materialparticles is used as the average hardness.

In a powder X-ray diffraction pattern, using CuKα rays, of the positiveelectrode active material particles 110, a ratio (A/B) of the half widthA of a peak obtained from a diffraction plane of Miller indices (003)with respect to the half width B of a peak obtained from a diffractionplane of Miller indices (104) is preferably about 0.7 or lower(typically, lower than 0.7), more preferably 0.65 or lower, and yet morepreferably 0.6 or lower (typically, lower than 0.6, for instance lowerthan 0.58). A lithium transition metal oxide exhibiting such a halfwidth ratio (A/B) has wider surfaces that allow for intercalation oflithium ions, and shorter ion diffusion distances within crystals, thana lithium transition metal oxide that exhibits a larger half width ratio(A/B). Therefore, a positive electrode active material having such aconfiguration allows enhancing, yet more effectively, the outputcharacteristic (in particular, output characteristic in a low SOCregion) of the lithium secondary battery. The lower limit of the halfwidth ratio (A/B) is not particularly restricted, but ordinarily thehalf width ratio (A/B) is preferably 0.35 or higher, (for instance, 0.4or higher), in terms of ease of production. A concern may arise, inbatteries provided with a positive electrode active material having anexcessively low half width ratio (A/B), in that metal elements in thepositive electrode active material may elute readily into theelectrolyte solution, for instance during high-temperature storage.Elution of such metal elements may be one cause of battery capacitydeterioration. From the viewpoint of the cycle characteristic at thetime of high-temperature storage, accordingly, the half width ratio(A/B) of the positive electrode active material is suitably 0.4 orhigher (for instance, 0.4<(A/B)), and preferably 0.5 or higher (forinstance, 0.5<(A/B)). For example, a positive electrode active materialthat satisfies 0.4≤(A/B)<0.7 can be preferably used herein in terms ofachieving a good balance between output characteristic and cyclecharacteristic. A positive electrode active material that satisfies0.4<(A/B)≤0.65 (and further 0.4<(A/B)<0.6, for instance 0.5≤(A/B)<0.6)allows achieving good results.

Positive electrode active material particles such as those describedabove can be produced in accordance with conventionally known productionmethods, for instance a method that involves precipitating, underappropriate conditions, a hydroxide of a transition metal comprised inthe lithium transition metal oxide that makes up the positive electrodeactive material particles (generation of a starting material hydroxide),from an aqueous solution that comprises at least one of the transitionmetal elements (preferably, all the metal elements, other than lithium,comprised in the oxide), and firing thereupon a mixture of thetransition metal hydroxide and a lithium compound.

In this case, generation of the starting material hydroxide may includea nucleation step of precipitating the transition metal hydroxide froman aqueous solution, under conditions of pH of 12 or higher and ammoniumion concentration of 25 g/L or lower; and a particle growth step ofgrowing the precipitated transition metal hydroxide, under conditions ofpH lower than 12 and ammonium ion concentration of 3 g/L or higher.Firing may be carried out such that the highest firing temperatureranges from 800° C. to 1100° C. Such a production method allows suitablyproducing positive electrode active material particles having thepierced hollow structure disclosed herein.

<<Lithium Secondary Batterγ>>

The present invention provides a lithium secondary battery having aconfiguration wherein an electrode assembly comprising a positiveelectrode and a negative electrode, and a nonaqueous electrolytesolution, are accommodated within a battery case. The positive electrodecomprises the positive electrode active material disclosed herein (i.e.a layered lithium transition metal oxide). The battery case comprises acurrent interrupt device that is activated when the internal pressure ofthe battery case rises.

Although not meant to be particularly limiting in any way, an example ofa lithium secondary battery of a form wherein a flat-wound electrodeassembly (wound electrode assembly) and a nonaqueous electrolytesolution are accommodated in a flat parallelepiped-shaped (box-shaped)container, will be explained in detail herein as a lithium secondarybattery according to one embodiment of the present invention.

The lithium secondary battery according to one embodiment of thetechnology disclosed herein has a configuration wherein, for instance asillustrated in FIG. 1 and FIG. 2, a wound electrode assembly 80 isaccommodated, together with a nonaqueous electrolyte solution, notshown, in a flat battery case 50 of flat parallelepiped (square) shapecorresponding to the shape of the wound electrode assembly 80. Thebattery case 50 comprises a battery case main body 52 having a flatparallelepiped shape (square shape) opened at the top end, and a lidbody 54 that plugs the opening of the battery case main body 52. Apositive electrode terminal 70 and a negative electrode terminal 72 forexternal connection are provided at the top face of the battery case 50(i.e. the lid body 54), such that part of these terminal juts out of thebattery through the lid body 54. A safety valve 55 for discharging tothe exterior gas that is generated inside the battery case is providedin the lid body 54.

A lithium secondary battery 100 having such a configuration can beconstructed, for instance, by accommodating the wound electrode assembly80 into the battery case 50, through the opening of the latter,attaching the lid body 54 to the opening of the battery case 50, andthereafter, injecting a nonaqueous electrolyte solution through anelectrolyte solution injection hole, not shown, that is provided in thelid body 54, followed by plugging of the injection hole for instance bywelding or the like. The sealing process of the battery case 50 and thearrangement (injection) process of the electrolyte solution can beaccomplished in accordance with methods identical to those of resortedto in the production of conventional lithium secondary batteries.

As illustrated in FIG. 2, an electrode assembly (wound electrodeassembly) 80 of a form resulting from flat winding of an elongatepositive electrode sheet 10 and an elongate negative electrode sheet 20,across an interposed an elongate separator sheet 40, is accommodated,together with a nonaqueous electrolyte solution not shown, inside thebattery case 50. A positive electrode collector plate 74 and a negativeelectrode collector plate 76 are respectively attached to an end of thepositive electrode sheet 10 (i.e. at a non-formation portion of thepositive electrode active material layer 14) and an end of the negativeelectrode sheet 20 (i.e. at a non-formation portion of the negativeelectrode active material layer 24). The positive electrode collectorplate 74 and the negative electrode collector plate 76 are electricallyconnected to above-described positive electrode terminal 70 and negativeelectrode terminal 72.

A current interrupt device 30 that is activated through a rise in theinternal pressure of the battery case 50 is provided inside the latter.The current interrupt device 30 is not limited to any specific shape,and it suffices that current interrupt device 30 be configured so that aconductive path (for instance, a charging path) from at least one of theelectrode terminals up to the wound electrode assembly 80 is cut offwhen the internal pressure of the battery case 50 rises. In theembodiment illustrated in FIG. 2, the current interrupt device 30, whichis provided between the wound electrode assembly 80 and the positiveelectrode terminal 70 that is fixed to the lid body 54, is configured sothat a conductive path from the positive electrode terminal 70 to thewound electrode assembly 80 is cut off in a case where the internalpressure of the battery case 50 rises. More specifically, the currentinterrupt device 30 may comprise, for instance, a first member 32 and asecond member 34. The current interrupt device 30 is configured so thatat least one from among the first member 32 and the second member 34deforms and moves away from the other member, so that the conductivepath is cut off as a result, in a case where the internal pressure ofthe battery case 50 rises. In the present embodiment, the first member32 is a deforming metal plate and the second member 34 is a connectingmetal plate that is joined to the deforming metal plate 32. Thedeforming metal plate (first member) 32 has, at the central portionthereof, a downward-curving arch shape, with a peripheral edge portionthereof being connected to the lower face of the positive electrodeterminal 70 via a collecting lead terminal 35. The leading end of acurved portion 33 of the deforming metal plate 32 is joined to the topface of the connecting metal plate 34. The lower face (rear face) of theconnecting metal plate 34 is joined to the positive electrode collectorplate 74, which is in turn connected to the positive electrode sheet 10of the electrode assembly 80. A conductive path becomes formed thus fromthe positive electrode terminal 70 to the wound electrode assembly 80.

The current interrupt device 30 comprises an insulating case 38 formedof plastic or the like. The insulating case 38, which is provided so asto surround the deforming metal plate 32, hermetically seals the topface of the latter. The internal pressure of the battery case 50 doesnot act on the top face of the hermetically sealed curved portion 33.The insulating case 38 has an opening through which the curved portion33 of the deforming metal plate 32 is inserted. The lower face of thecurved portion 33 is exposed to the interior of the battery case 50through this opening. The internal pressure of the battery case 50 actson the lower face of the curved portion 33 that is exposed to theinterior of the battery case 50. When the internal pressure of thebattery case 50 rises, the internal pressure acts on the lower face ofthe curved portion 33 of the deforming metal plate 32 of the currentinterrupt device 30 thus configured, whereupon the curved portion 33that is curved downward is pushed up. The upward push of the curvedportion 33 increases as the internal pressure of the battery case 50rises. When the internal pressure of the battery case 50 exceeds a setpressure, the curved portion 33 flips vertically and deforms so as tocurve upward. A junction 36 between the deforming metal plate 32 and theconnecting metal plate 34 becomes cut off as a result of the deformationof the curved portion 33. The conductive path from the positiveelectrode terminal 70 to the electrode assembly 80 becomes cut off as aresult, and the overcharge current is interrupted.

The current interrupt device 30 is not limited to being provided on thepositive electrode terminal 70 side, and may be provided on the negativeelectrode terminal 72 side. The current interrupt device 30 is notlimited to mechanical cut-off upon deformation of the above-describeddeforming metal plate 32, and, for instance, an external circuit can beprovided, as the current interrupt device, such that the chargingcurrent is cut off when the internal pressure of the battery case 50, asdetected by a sensor, exceeds a set pressure.

FIG. 3 is a diagram illustrating schematically an elongate sheetstructure (electrode sheet) at a stage prior to assembling the woundelectrode assembly 80. In the wound positive electrode sheet 10, thepositive electrode active material layer 14 is formed along thelongitudinal direction, on one or both faces (typically, both faces) ofthe elongate positive electrode collector 12, such that the positiveelectrode active material layer 14 is not provided (or is removed) andthe positive electrode collector 12 is exposed, at a first edge sectionalong the longitudinal direction of the positive electrode sheet 10.Similarly, the wound negative electrode sheet 20 is formed along thelongitudinal direction, on one or both faces (typically, both faces) ofthe elongate negative electrode collector 22, such that the negativeelectrode active material layer 24 is not provided (or is removed) andthe negative electrode collector 22 is exposed, at a first edge sectionalong the longitudinal direction of the negative electrode sheet 20. Awound electrode assembly can be produced then by stacking the positiveelectrode sheet 10 and the negative electrode sheet 20, together withthe elongate separator sheet 40, and winding then the resulting stack inthe longitudinal direction. The positive electrode sheet 10 and thenegative electrode sheet 20 are superposed slightly offset from eachother, in the width direction, in such a manner that a positiveelectrode active material layer non-formation portion of the positiveelectrode sheet 10 and a negative electrode active material layernon-formation portion of the negative electrode sheet 20 just beyondboth respective sides of the separator sheet 40, in the width direction.The resulting wound electrode assembly is squashed from the sides, toyield thereby a flat wound electrode assembly 80.

<Positive Electrode Sheet 10>

The positive electrode sheet 10 of the lithium secondary batterydisclosed herein is provided with the positive electrode collector 12,and the positive electrode active material layer 14 comprising at leasta positive electrode active material and being formed on the positiveelectrode collector. The positive electrode active material layer 14comprises any one of the positive electrode active materials disclosedherein, and, as needed, for instance a conductive material such that theforegoing are fixed to the positive electrode collector 12.

Such a positive electrode sheet 10 can be preferably produced byapplying (typically, by coating) a paste-like or slurry-like composition(dispersion for forming a positive electrode active material layer)resulting from dispersing a positive electrode active material, and forinstance a conductive material, a binder and so forth that are used asneeded, in an appropriate solvent, onto the positive electrode collector12, followed by drying. Materials already described above can beappropriately selected and used as the positive electrode activematerial. The solvent that can be used may be an aqueous solvent or anorganic solvent. For instance, N-methyl-2-pyrrolidone (NMP) can be usedherein.

A conductive member comprising a metal of good conductivity (forinstance, aluminum, nickel, titanium, stainless steel or the like) canbe suitably used in the positive electrode collector 12. The collectormay adopt various shapes, in accordance with, for instance, the shape ofthe battery that is constructed, and is therefore not particularlylimited. The collector may be, for instance, a rod-like body, plate-likebody, a foil-like body or a mesh-like body. A foil-like body is mainlyresorted to in batteries that are provided with a wound electrodeassembly. The thickness of the foil-like collector is not particularlylimited, and may be set to range from about 5 μm to 50 μm (morepreferably, from 8 μm to 30 μm), in terms of a trade-off betweencapacity density of the battery and collector strength.

A carbon material can be typically used as the conductive material.Specific examples thereof include, for instance, one, two or more typesselected from among carbon materials such as carbon black (for instance,acetylene black, Ketjen black), coke, activated carbon, graphite(natural graphite, synthetic graphite) and carbon fibers (PAN-basedcarbon fibers, pitch-based carbon fibers), carbon nanotubes, fullerenes,graphene and the like. Carbon black (typically, acetylene black) can beappropriately used among the foregoing.

As the binder there can be used a polymer that can dissolve or dispersein the solvent that is used. In compositions that utilize a nonaqueoussolvent, for instance polyvinylidene fluoride (PVdF), polyvinylidenechloride (PVdC) or the like can be preferably used. In compositions thatutilize an aqueous solvent there can be preferably used a cellulosicpolymer, for instance carboxymethyl cellulose (CMC; typically a sodiumsalt thereof), hydroxypropyl methyl cellulose (HPMC) or the like;polyvinyl alcohol (PVA); a fluororesin such as polytetrafluoroethylene(PTFE) or the like; or a rubber such as styrene butadiene rubber (SBR)or the like.

The proportion of the positive electrode active material in the positiveelectrode active material layer 14 as a whole is, appropriately, about50% by mass or higher (typically, in the range 50% by mass to 95% bymass); preferably, the proportion ranges ordinarily from about 70% bymass to 95% by mass. In a case where a conductive material is used, theproportion of the conductive material in the positive electrode activematerial layer 14 as a whole can be set to range, for instance, fromabout 2% by mass to 20% by mass; preferably, the proportion is set torange ordinarily from about 2% by mass to 15% by mass. In a case where abinder is used, the proportion of the binder in the positive electrodeactive material layer 14 as a whole can be set to range, for instance,from about 0.5% by mass to 10% by mass; preferably, the proportion isset to range ordinarily from about 1% by mass to 5% by mass.

The mass of the positive electrode active material layer 14 that isprovided per unit surface area of the positive electrode collector 12(total mass on both faces in a configuration with the positive electrodeactive material layer 14 on both faces of the positive electrodecollector 12) is appropriately set to range, for instance, from about 5mg/cm² to 40 mg/cm² (typically, from about 10 mg/cm² to 20 mg/cm²). Thedensity of the positive electrode active material layer 14 may be set torange, for instance, from about 1.5 g/cm³ to 4 g/cm³ (typically, fromabout 1.8 g/cm³ to 3 g/cm³). Suitable conductive paths can be formed inthe positive electrode active material by prescribing the density of thepositive electrode active material layer 14 to lie in the above ranges.As a result, the resistance of the positive electrode active materiallayer 14 can be reduced, which allows realizing high batteryperformance. It becomes also possible to maintain proper voids withinthe positive electrode active material layer 14, and to elicitsufficient impregnation by the electrolyte solution. As a result,excellent battery performance (for instance, energy density andinput-output density) can be brought out during ordinary use, and abattery can be suitably realized that allows activating the CID throughprompt generation of gas during overcharge.

<Negative Electrode Sheet 20>

The negative electrode sheet 20 of the lithium secondary batterydisclosed herein is provided with the negative electrode collector 22,and the negative electrode active material layer 24 comprising at leasta negative electrode active material and formed on the negativeelectrode collector. The negative electrode active material layer 24comprises at least a negative electrode active material, and is fixed tothe negative electrode collector 22. Such a negative electrode sheet 20can be produced more preferably by applying (typically, by coating) apaste-like or slurry-like composition (dispersion for forming a negativeelectrode active material layer) resulting from dispersing, in anappropriate solvent, a negative electrode active material and forinstance a binder and so forth used as needed, onto the negativeelectrode collector 22, followed by drying. A conductive materialcomprising a metal of good conductivity (for instance, copper, nickel,titanium, stainless steel or the like) is used preferably as thenegative electrode collector 22. The shape of the negative electrodecollector 22 may be identical to the shape of the positive electrodecollector. The solvent that can be used may be an aqueous solvent or anorganic solvent. For instance, water can be used herein.

One, two or more types of known materials that can be used as negativeelectrode active materials of lithium secondary batteries can likewisebe utilized herein, without particular limitations, as the negativeelectrode active material. Herein there can be used, although notparticularly limited thereto, for instance a carbon material such asnatural graphite (black lead), synthetic graphite, hard carbon (hardlygraphitizable carbon), soft carbon (easily graphitizable carbon), carbonblack or the like; metal oxide materials such as silicon oxide, titaniumoxide, vanadium oxide, iron oxide, cobalt oxide, nickel oxide, niobiumoxide, tin oxide, lithium silicon complex oxides, lithium titaniumcomplex oxides (lithium titanium composite oxide: LTO, for instanceLi₄Ti₅O₁₂, LiTi₂O₄ or Li₂Ti₃O₇), lithium vanadium complex oxides,lithium manganese complex oxides, lithium tin complex oxides and thelike; metal nitride materials such as lithium nitride, lithium cobaltcomplex nitrides, lithium nickel complex nitrides and the like; ormetallic material comprising metals such as tin, silicon, aluminum,zinc, lithium or the like, or metal alloys having the foregoing metalelements as main constituents.

As the binder there can be used an appropriate binder from among thepolymer materials having been exemplified as binders of the positiveelectrode active material layer above. Specific examples include, forinstance, styrene butadiene rubber (SBR), polyvinylidene fluoride(PVdF), polytetrafluoroethylene (PTFE) and the like. Besides the binder,various additives such as dispersants, conductive materials and the likecan be used as appropriate.

The proportion of the negative electrode active material in the negativeelectrode active material layer 24 as a whole is appropriately set toabout 50% by mass or higher, and preferably to range from 90% by mass to99% by mass (for instance, 95% by mass to 99% by mass). In a case wherea binder is used, the proportion of the conductive material in thenegative electrode active material layer 24 as a whole can be set torange, for instance, from about 1% by mass to 10% by mass; preferably,the proportion is set to range ordinarily from about 1% by mass to 5% bymass.

The mass of the negative electrode active material layer 24 that isprovided per unit surface area of the negative electrode collector 22(total mass on both faces of the negative electrode collector 22 in acase of structure having the negative electrode active material layer 24on both faces of the negative electrode collector 22) is appropriatelyset to range, for instance, from about 5 mg/cm² to 20 mg/cm² (typically,from about 5 mg/cm² to 10 mg/cm²). The density of the negative electrodeactive material layer 24 may be set to range, for instance, from about0.5 g/cm³ to 2 g/cm³ (typically, from about 1 g/cm³ to 1.5 g/cm³).Diffusion resistance of lithium ions can be kept low, while maintaininga desired capacity, by prescribing the density of the negative electrodeactive material layer 24 to lie in the above ranges. As a result, itbecomes possible to realize yet higher battery performance (forinstance, output characteristic and energy density).

<Separator 40>

Separators identical to separators for ordinary lithium secondarybatteries can be used herein, without particular limitations, as theseparator 40. For instance, there can be used porous sheets, nonwovenfabrics or the like made up of a resin such as polyethylene (PE),polypropylene (PP), polyester, cellulose, polyamide or the like.Suitable examples thereof include porous sheets (micro-porous resinsheets) having a single-layer or multilayer structure the mainconstituent whereof is one, two or more types of polyolefin resin.Sheets that can be appropriately used herein include PE sheets, PPsheets, and sheets having a three-layer structure (PP/PE/PP structure)in which a PP layer is overlaid on both sides of a PE layer. A porousheat-resistant layer may be provided on one or both faces (typically,one face) of the above porous sheet. The porous heat-resistant layer cancomprise for instance an inorganic material (an inorganic filler such asalumina particles or the like can be preferably used herein) and abinder. Alternatively, the porous heat-resistant layer may compriseinsulating resin particles (for instance, particles of polyethylene,polypropylene or the like).

<Battery Case 50>

Examples of the material of the battery case 50 include metallicmaterials such as aluminum, steel or the like, and resin materials suchas polyphenylene sulfide resins, polyimide resins and the like. Acomparatively lightweight metal (for instance, aluminum or aluminumalloy) can be preferably used among the foregoing in terms of enhancingheat dissipation and increasing energy density. The shape (containeroutline) of the battery case 50 is not particularly limited, and may be,for instance, a circular shape (cylindrical shape, coin shape, buttonshape), a hexahedral shape (rectangular parallelepiped (prismaticshape), cubic shape) or a bag-body shape, or a shape resulting fromworking or deforming the foregoing.

<Nonaqueous Electrolyte Solution>

As the nonaqueous electrolyte solution there can be preferably used anonaqueous electrolyte solution resulting from dissolving or dispersinga supporting salt (a lithium salt in lithium secondary batteries) in anonaqueous solvent. A salt similar to that of ordinary lithium secondarybatteries can be used as appropriate as the supporting salt. Forinstance, a lithium salt such as LiPF₆, LiBF₄, LiClO₄, LiAsF₆,Li(CF₃SO₂)₂N, LiCF₃SO₃ or the like can be used as the lithium salt. Sucha supporting salt can be used singly or in combinations of two or moretypes. In particular, LiPF₆ is a preferred example of the supportingsalt. Preferably, the nonaqueous electrolyte solution is prepared insuch a manner that the concentration of the supporting salt lies in therange 0.7 mol/L to 1.3 mol/L.

One, two or more types of organic solvent used in ordinary lithiumsecondary batteries can be selected and used, as appropriate, as theabove nonaqueous solvent. Examples of particularly preferred nonaqueoussolvents include, for instance, ethylene carbonate (EC), diethylcarbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC),vinylene carbonate (VC), propylene carbonate (PC) and the like. Forinstance, a mixed solvent comprising EC, DMC and EMC at a volume ratio3:4:3 can be appropriately used herein.

In one preferred implementation, the nonaqueous electrolyte solutioncomprises an additive (gas generating agent) that can generate a gas bydecomposing when a predetermined battery voltage is exceeded. As the gasgenerating agent there can be used, without any particular limitations,one, two or more compounds selected from among compounds that are usedin similar applications, so long as the compound can generate a gas bydecomposing when a predetermined battery voltage is exceeded(specifically, compounds the oxidation potential whereof (vs. Li/Li⁺) isequal to or greater than the charging upper limit potential of thepositive electrode, such that the compound can generate a gas bydecomposing when that potential is exceeded and an overcharge state isaccordingly brought about). Specific examples of the gas generatingagent include, for instance, aromatic compounds such as biphenylcompounds, alkylbiphenyl compounds, cycloalkylbenzene compounds,alkylbenzene compounds, organophosphorus compounds, fluorineatom-substituted aromatic compounds, carbonate compounds, cycliccarbamate compounds, alicyclic hydrocarbons and the like.

More specific compounds include, for instance, biphenyl,cyclohexylbenzene, 1-fluoro-2-cyclohexylbenzene,1-fluoro-3-cyclohexylbenzene, 1-fluoro-4-cyclohexylbenzene,1-bromo-4-cyclohexylbenzene, trans-butylcyclohexylbenzene,cyclopentylbenzene, tert-butylbenzene, tert-pentylbenzene,1-fluoro-4-tert-butylbenzene, 1-chloro-4-tert-butylbenzene,1-bromo-4-tert-butylbenzene, tert-pentylbenzene,1-fluoro-4-tert-pentylbenzene, 1-chloro-4-tert-pentylbenzene,1-bromo-4-tert-pentylbenzene, tert-aminobenzene, terphenyl,2-fluorobiphenyl, 3-fluorobiphenyl, 4-fluorobiphenyl,4,4′-difluorobiphenyl, o-cyclohexylfluorobenzene,p-cyclohexylfluorobenzene, tris-(t-butylphenyl)phosphate, phenylfluoride, 4-fluorophenyl acetate, diphenyl carbonate, methylphenylcarbonate, bis-tert-butylphenyl carbonate, diphenyl ether, dibenzofuranand the like.

For instance, biphenyl (BP) or cyclohexylbenzene (CHB) can be preferablyused in batteries where the charging upper limit potential of thepositive electrode (vs. Li/Li⁺) is set to range from about 4.0 V to 4.3V. These gas generating agents have an oxidation potential (vs. Li/Li⁺)of about 4.5 V to 4.6 V. Specifically, these gas generating agents havean oxidation potential higher by about 0.2 V to 0.6 V than the chargingupper limit potential of the positive electrode, and, accordingly, cangenerate a gas (typically, hydrogen gas) promptly by undergoing rapidoxidative decomposition, in the positive electrode, at an early stage ofovercharge. These compounds form readily conjugated systems and exchangeelectrons easily, and exhibit accordingly good reactivity (areoxidatively polymerizable). Therefore, the current interrupt device canbe activated quickly and accurately, and the reliability of the batteryduring overcharge can be enhanced yet further.

The concentration of the gas generating agent in the nonaqueouselectrolyte solution is not particularly limited, but is appropriatelyabout 0.1% by mass or higher, and preferably 0.5% by mass or higher,with respect to 100% by mass of the nonaqueous electrolyte solution,from the viewpoint of securing a sufficient amount of gas for activatingthe overcharge prevention mechanism. The gas generating agent, however,can give rise to resistance components in cell reactions, and hence aconcern of reduced input-output characteristic arises in a case where anexcessive amount of gas generating agent is added. From that point ofview, a suitable addition amount of gas generating agent is about 5% bymass or less, and is preferably set to 4% by mass or less. Ordinarily,setting a range of 0.1% by mass to 5% by mass is appropriate;preferably, the addition amount range is for instance set to 0.1% bymass to 4% by mass (preferably, 0.5% by mass to 3% by mass, inparticular 0.5% by mass to 2% by mass).

The nonaqueous electrolyte solution can contain components other thanthe supporting salt, gas generating agent and nonaqueous solventdescribed above, so long as the effect of the present invention is notsignificantly impaired thereby. Any such component can be used for one,two or more purposes from among increasing the gas generation amountduring overcharge, enhancing output performance and storability (forinstance, suppressing drops in capacity during storage), enhancing thecycle characteristic, and enhancing the initial charge and dischargeefficiency of the battery. Examples of such components include variousadditives, for instance film-forming agents such as lithium bisoxalateborate (LiBOB), vinylene carbonate (VC), vinyl ethylene carbonate (VEC)or the like, dispersants such as carboxymethyl cellulose (CMC) or thelike, and thickeners.

Various examples pertaining to the present invention will be explainedbelow, but the present invention is not meant to be limited to or by thematter illustrated in the specific examples.

As positive electrode active materials, firstly nine types of lithiumtransition metal complex oxide were prepared that had differentcomposition and/or properties, as given in Table 1. The results of SEMobservation performed on the positive electrode active materialsrevealed that all positive electrode active materials in Example 1 toExample 9 had a pierced hollow structure.

As an example, FIG. 5 and FIG. 6 illustrate SEM observation images of apositive electrode active material according to Example 1. FIG. 5 is aSEM observation image of the resulting positive electrode activematerial particles, and FIG. 6 is a cross-sectional SEM observationimage of a cross section cut through embedding and grinding of thepositive electrode active material particles. The positive electrodeactive material particles prepared herein had the form of secondaryparticles 110 resulting from aggregation of primary particles 112, andcomprised a distinct shell section 115 and hollow section 116. Asillustrated in FIG. 6, it was observed that an average of one or morethrough-holes 118 per particle were formed in the shell section 115,with dense sintering of the shell section at portions other than thethrough-holes. Such observation, performed at ten arbitrary sites,revealed that the proportion (particle porosity of cross-sectional arearatio) of the hollow section 116 was about 23%, the thickness of theshell section 115 (average value of the shortest distance T(k) from anyposition k on the inner surface 115 a of the shell section 115 up to theouter surface 115 b of the shell section 115) was about 1.2 μm, and themajor axis L1 of the primary particles 112 was 0.7 μm. Measurements ofthe hardness and half width ratio (A/B) of the resulting particles,performed in accordance with the methods already described above,yielded an average hardness in the range 0.5 MPa to 100 MPa, and a halfwidth ratio (A/B) in the range 0.4 to 0.7.

TABLE 1 D₅₀ (D₉₀ − Tap Crystallite Average composition (μm) D₁₀)/D₅₀density(g/cm³) size (Å) Example 1Li_(1.14)Ni_(0.34)Co_(0.33)Mn_(0.33)Ca_(0.0002)O₂ 5.7 0.42 1.88 1073Example 2 Li_(1.14)Ni_(0.34)Co_(0.33)Mn_(0.33)Ca_(0.0002)O₂ 6.1 0.441.82 980 Example 3 Li_(1.14)Ni_(0.34)Co_(0.33)Mn_(0.33)Ca_(0.0001)O₂ 5.70.42 1.82 1060 Example 4Li_(1.14)Ni_(0.34)Co_(0.33)Mn_(0.33)Ca_(0.0002)O₂ 8.1 0.56 1.91 903Example 5 Li_(1.14)Ni_(0.34)Co_(0.33)Mn_(0.33)Ca_(0.0002)O₂ 8 0.55 2.41860 Example 6 Li_(1.14)Ni_(0.34)Co_(0.33)Mn_(0.33)Ca_(0.0001)O₂ 5.740.42 1.8 1050 Example 7Li_(1.14)Ni_(0.34)Co_(0.33)Mn_(0.33)Ca_(0.0002)O₂ 4 0.39 1.6 1210Example 8 Li_(1.14)Ni_(0.34)Co_(0.33)Mn_(0.33)Ca_(0.0003)O₂ 5.7 0.421.82 980 Example 9 Li_(1.14)Ni_(0.34)Co_(0.33)Mn_(0.33)Ca_(0.0002)O₂ 90.69 2.55 920

A laminate sheet-type cell (lithium secondary battery) was constructedusing each of the positive electrode active material particles accordingto Example 1 to Example 9, and the characteristics of each cell wasevaluated.

The positive electrode active material particles (LNCMC) given in Table1, acetylene black (AB) as a conductive material, and polyvinylidenefluoride (PVdF) as a binder were charged in a kneader, to a mass ratioof LNCMC:AB:PVdF=90:8:2 of the foregoing materials, and the whole waskneaded, while under adjustment of the viscosity with N-methylpyrrolidone (NMP), to a solids concentration of 50% by mass, to preparerespective positive electrode active material slurries. Both faces of anelongate sheet-like aluminum foil about 15 μm thick (positive electrodecollector) were coated by roller coating, to a basis weight of 20 mg/cm²(solids basis), with respective bands of each slurry, followed by drying(drying temperature 80° C., for 5 minutes), to produce thereby arespective positive electrode sheet (Example 1 to Example 9) in which apositive electrode active material layer was provided on both faces ofthe positive electrode collector. The positive electrode sheets wererolled in a roll press, to adjust the thickness of the sheets to 130 μmand the electrode density to 2.8 g/cm³.

A negative electrode active material (natural graphite: C, averageparticle size 5 μm), styrene butadiene rubber (SBR) as a binder, andcarboxymethyl cellulose (CMC) as a thickener, were charged in a kneaderto a mass ratio of C:SBR:CMC=98:1:1 of the foregoing materials. Thewhole was kneaded, while under adjustment of the viscosity withdeionized water, to a solids concentration of 45% by mass, to prepare anegative electrode active material slurry. Both faces of an elongatesheet-like elongate copper foil 10 μm thick (negative electrodecollector) were coated by roller coating, to a basis weight of 14 mg/cm²(solids basis), with respective bands of the slurry, followed by drying(drying temperature 100° C., for 5 minutes), to produce a negativeelectrode sheet in which a negative electrode active material layer wasprovided on both faces of the negative electrode collector. The negativeelectrode sheet was rolled in a roll press, to adjust the thickness ofthe sheet to 100 μm and the electrode density to 1.4 g/cm³.

Each positive electrode sheet and the negative electrode sheet thusproduced were arranged opposing each other across a separator (theseparator used herein had a three-layer structure (total thickness 20μm, porosity 48 vol %) of polypropylene (PP) overlaid on both faces ofpolyethylene (PE), and provided, at the surface, with a porousheat-resistant layer having alumina as an main constituent), to producea respective stacked electrode assembly. A positive electrode terminaland a negative electrode terminal were respectively attached to thepositive electrode collector (uncoated section of the positive electrodeactive material layer) and the negative electrode collector (uncoatedsection of the negative electrode active material layer), exposed at theends of each electrode assembly. The electrode assembly was accommodatedinside a laminate film, and the whole was dried under reduced pressureand high temperature, to remove moisture. Thereafter, a nonaqueouselectrolyte solution (the nonaqueous electrolyte solution used hereinwas obtained by dissolving LiPF₆ as a supporting salt, to aconcentration of 1.1 mol/L, in a mixed solvent of ethylene carbonate(EC), dimethyl carbonate (DMC) and ethyl methyl carbonate (EMC) at avolume ratio of EC:DMC:EMC=3:4:3, and by further dissolving a gasgenerating agent (biphenyl), at a concentration of 2% by weight) wasinjected through the opening of the laminate film, and the opening wassealed. The lithium secondary batteries of Example 1 to Example 9, inwhich only the positive electrode active material varied, were thusconstructed.

TABLE 2 Positive electrode active material Evaluation results particles.Gas generation Initial Positive electrode Ca addition D₅₀ Tap densityamount capacity resistance ratio (μm) (g/cm³) (relative value) (mAh/g)(mΩ) Example 1 0.0002 5.7 1.88 100 159 3.3 Example 2 0.002 6.1 1.82 167150.5 2.3 Example 3 0.001 5.7 1.82 147 154.5 3.1 Example 4 0.0002 8.11.91 113 160.2 2.9 Example 5 0.0002 8 2.41 167 153.7 3.9 Example 60.0001 5.74 1.8 87 159.6 3.2 Example 7 0.0002 4 1.6 80 159.2 2.5 Example8 0.003 5.7 1.82 173 145.1 3.3 Example 9 0.0002 9 2.55 180 158.3 4.9

<Conditioning>

The constructed batteries were subjected to conditioning. Herein 3cycles of charge and discharge were performed, each cycle involving (1)through (4) below.

(1) Constant current charging (CC charging) to 4.1 V at a rate of 1 C(50 mA).

(2) Pause of five minutes.

(3) Constant current discharge (CC discharge) to 3.0 V at a rate of 1 C(50 mA).

(4) Pause of five minutes.

<Measurement of Rated Capacity (Initial Capacity)>

Each battery after conditioning was charged and discharged over avoltage range from 3.0 V to 4.2 V, according to (1) to (4) below, in atemperature environment at 25° C., and initial capacity was checked.

(1) The battery was charged at constant current (CC charging) at a rateof 1 C (50 mA), until the battery voltage reached 4.2 V; thereafter, thebattery was charged at constant voltage (CV charging), until the currentreached a rate of 0.01 C (0.5 mA).

(2) Pause of 1 hour.

(3) The battery was CC-discharged at a rate of 1 C (50 mA) until thebattery voltage reached 3.0 V; thereafter, the battery was discharged atconstant voltage (CV discharge) until the current reached a rate of 0.01C (0.5 mA).

(4) Pause of 5 minutes.

The resulting discharge capacity (sum of the products of current valuesand voltage values) was taken as the rated capacity (initial capacity).The results are set out in the column “Initial capacity” of Table 2.FIG. 7 illustrates a relationship between the Ca addition ratio of thepositive electrode active material and the initial capacity of batteriesin Example 1 to Example 3, Example 6 and Example 8, in which tap densityis roughly equivalent to 1.8 to 1.9.

As FIG. 7 reveals, a trend is observed whereby initial capacity, i.e.the energy density of the battery, decreases as the Ca addition ratioincreases. For instance, an energy density of 150 mAh/g or higher can berealized in batteries that require high energy density, such as thosebatteries that are used in, for instance, power sources for driving avehicle. From the above point of view, it was found that the Ca additionratio (denoted by β in Formula (I) above) of the positive electrodeactive material was typically 0.0025 or lower, for instance lower than0.0025, preferably 0.002 or lower, and particularly preferably lowerthan 0.002.

<Measurement of the Resistance of the Positive Electrode>

The resistance of the positive electrode was measured next in a 25° C.temperature environment. Specifically, the battery was firstly chargedat constant current 1 C (50 mA) until the voltage across terminals ofthe positive and negative electrodes was 4.1 V. Thereafter, the batterywas charged at constant voltage for 3 hours, to adjust the battery to afully charged state. The resistance of the positive electrode wasmeasured in accordance with an AC impedance measurement method, underthe conditions below. An equivalent circuit was fitted to a resultingCole-Cole plot (also referred to as Nyquist plot), to work out theresistance (mΩ) of the positive electrode. The results are given in thecolumn “Battery resistance (mΩ)” of Table 2. FIG. 8 illustrates arelationship between battery resistance and the tap density of thepositive electrode active material, for the batteries of Example 1,Example 4, Example 5, Example 7 and Example 9, having the same Caaddition ratio of 0.0002.

As FIG. 8 reveals, a trend was observed wherein the higher the tapdensity of the positive electrode active material, the higher theresistance of the positive electrode is. Preferably, the resistance ofthe positive electrode is 4 mΩ or smaller in batteries that require ahigh energy density or high output density, such as those that areutilized, for instance, in power sources for driving a vehicle. From theabove point of view, it was found that the tap density of the positiveelectrode active material is lower than 2.55, typically 2.5 or lower,for instance lower than 2.5, preferably 2.45 or lower, and particularlypreferably lower than 2.45.

<Overcharge Test>

Gas generation amounts were measured next in a 25° C. temperatureenvironment. Specifically, the thickness of each battery prior to anovercharge test (i.e. the thickness in the stacking direction of theelectrode assembly) was measured first using a rotary caliper.Thereafter, the battery was CC-charged at 1 C (50 mA) until the voltageacross terminals in the positive and negative electrodes reached 4.1 V.Thereafter, the battery was CV-charged for 3 hours, to adjust thebattery to a fully charged state. The battery in this fully chargedstate was further CC-charged at 2 C (100 mA) until the integratedcurrent reached 150 mA (i.e. overcharge state). The thickness of thebattery in the overcharge state was measured. The increment in thicknessderived from generation of gas during overcharge was calculated bysubtracting the thickness of the battery (cm) before the overcharge testfrom the thickness (cm) of the battery in the overcharge state. Theobtained result was divided by a thickness corresponding to the amountof gas necessary for activating the CID, and the result was multipliedby 100, to calculate a relative value. The results are given in thecolumn “Gas generation amount” of Table 2. The larger this value, thegreater is the gas generation amount during overcharge as denoted by thevalue. FIG. 7 illustrates the relationship between gas generation amountand Ca addition ratio, and FIG. 8 illustrates the relationship betweengas generation amount and tap density.

As FIG. 7 reveals, a trend was observed wherein the gas generationamount during overcharge increases as the Ca addition ratio becomesgreater. This can be ascribed to the enhanced reactivity duringovercharge that results from incorporating Ca as a structural element.It was found therefore that the Ca addition ratio of the positiveelectrode active material (β in Formula (I) above) is typically 0.0002or higher, and preferably higher than 0.0002, from the viewpoint ofreliability during overcharge. A desired amount of gas can be stablyobtained during overcharge when the above value of 13 is satisfied.

Accordingly, it was found that value of β in Formula (I) above istypically 0.0002≤β≤0.0025, for instance 0.0002≤β<0.0025, preferably0.0002≤β≤0.002, and particularly preferably 0.0002≤β<0.002, from theviewpoint of achieving, at a yet higher level, both battery performance(for instance, energy density, input-output density) and reliabilityduring overcharge. These results bear out the technical significance ofthe present invention.

As FIG. 8 further reveals, a trend was found wherein the higher the tapdensity of the positive electrode active material, the greater is thegas generation amount during overcharge. This arises conceivably fromthe fact that discharge paths for the gas generated during overchargecan be secured through an increase in the tap density, whereby the gascan be discharged quickly out of the electrode assembly. Therefore, itwas found that the tap density of the positive electrode active materialis 1.8 or higher, typically 1.85 or higher, for instance higher than1.85, preferably 1.88 or higher, and particularly preferably higher than1.88, from the viewpoint of reliability during overcharge.

It was accordingly found that the tap density of the positive electrodeactive material is typically in the range 1.8 to 2.55, for instance inthe range 1.8 to 2.5, preferably in the range 1.85 to 2.5, andparticularly preferably in the range 1.88 to 2.45, from the viewpoint ofcombining, at a yet higher level, battery performance (for instance,energy density and input-output density) with reliability duringovercharge. Such results bear out the effect of the present invention.

The present invention has been explained in detail above, but the aboveembodiments are merely illustrative, and the invention disclosed hereinincludes all manner of variations and modifications of the specificexamples described above.

INDUSTRIAL APPLICABILITY

By virtue of the excellent performance exhibited by the lithiumsecondary battery provided in accordance with the technology disclosedherein, the lithium secondary battery can be used as a lithium secondarybattery for various applications. Among such applications, the lithiumsecondary battery can be suitably used as a power source for motors(electric motors) that are installed in vehicles such as automobiles.Such a lithium secondary battery may be used in the form of an assembledbattery resulting from connecting in series and/or in parallel aplurality of such lithium secondary batteries. Therefore, the technologydisclosed herein allows providing a vehicle (typically, an automobile,in particular an automobile that is provided with an electric motor, forinstance a hybrid automobile, a plug-in hybrid automobile, an electricautomobile, a fuel cell automobile, an electric scooter, apower-assisted bicycle, an electric wheelchair, an electric railway andthe like) that comprises such a lithium secondary battery (which may bein the form of an assembled battery), as a power source.

REFERENCE SIGNS LIST

-   -   10 positive electrode sheet (positive electrode)    -   12 positive electrode collector    -   14 positive electrode active material layer    -   20 negative electrode sheet (negative electrode)    -   22 negative electrode collector    -   24 negative electrode active material layer    -   30 current interrupt device (CID)    -   32 deformation metal plate (conduction member; first member)    -   33 curved portion    -   34 connection metal plate (conduction member; second member)    -   35 collecting lead terminal    -   36 junction    -   38 insulating case    -   40 separator sheet (separator)    -   50 battery case    -   52 battery case main body    -   54 lid body    -   55 safety valve    -   70 positive electrode terminal    -   72 negative electrode terminal    -   74 positive electrode collector plate    -   76 negative electrode collector plate    -   80 wound electrode assembly    -   100 lithium secondary battery    -   110 positive electrode active material particles (positive        electrode active material)    -   112 primary particle    -   115 shell section    -   115 a inner surface of shell section    -   115 b outer surface of shell section    -   116 hollow section    -   118 through-hole

The invention claimed is:
 1. A particulate positive electrode activematerial that is used in a lithium secondary battery, comprising: alithium transition metal oxide of a layered crystal structure,comprising Ni, Co, Mn and Ca as structural elements and beingrepresented by the following formula (I):Li_(1+α)Ni_(x)Co_(y)Mn_(z)Ca_(β)O₂  (I) wherein, in formula (I),−0.05≤α≤0.2, x+y+z+β≈=1, 0.3≤x≤0.7, 0.1≤y≤0.4, 0.1≤z≤0.4, and0.001≤β≤0.002, wherein Ca is a substitutional element in the crystalstructure of the lithium transition metal oxide, the tap density of thepositive electrode active material ranges from 1.8 g/cm³ to 2.5 g/cm³,and in a volume-basis particle size distribution measured on the basisof a laser diffraction/light scattering method: an average particle sizeD₅₀ corresponding to a cumulative 50% from the fine particle side of theparticle size distribution ranges from 5 μm to 9 μm, and a particle sizeD₁₀ corresponding to a cumulative 10% from the fine particle side of theparticle size distribution, a particle size D₉₀ corresponding to acumulative 90% from the fine particle side of the particle sizedistribution, and said average particle size D₅₀ satisfy the followingrelationship: (D₉₀−D₁₀)/D₅₀≤0.7.
 2. The positive electrode activematerial according to claim 1, wherein said positive electrode activematerial is a hollow structure having a shell section made up of thelithium transition metal oxide of the layered crystal structure, and ahollow section formed inside the shell section, and the thickness ofsaid shell section, on the basis of an electron microscope observation,ranges from 0.1 μm to 2 μm.
 3. The positive electrode active materialaccording to claim 2, wherein said positive electrode active materialhas a through-hole that runs through said shell section.
 4. The positiveelectrode active material according to claim 1, wherein a crystallitesize r of said positive electrode active material, based on X-raydiffraction, ranges from 0.05 μm to 0.2 μm.
 5. A lithium secondarybattery in which an electrode assembly including a positive electrodeand a negative electrode, and an nonaqueous electrolyte solution, areaccommodated inside a battery case, wherein said battery case has acurrent interrupt device that is activated when an internal pressure ofthe battery case rises; and said positive electrode has the positiveelectrode active material according to claim 1.